Contrast enhancement filters having neutral density over the visible spectrum are known to enhance the contrast of self-luminous electronic displays (e.g., cathode ray tubes or CRTs) which often suffer from loss of contrast when viewed in bright ambient light. By placing a contrast enhancement filter between the viewer and the display, light reflected off the face of the CRT passes through the contrast enhancement filter twice whereas light emitted by the CRT passes through the optical filter only once. As a result, the contrast enhancement filter reduces the intensity of the reflected light relative to the intensity of the light emitted by the CRT thereby enhancing the contrast of the display.
When displays are viewed under varying conditions of ambient light, variable density contrast enhancement filters are useful since the ability to vary the density of the filter allows the user to control the contrast enhancement and glare reduction provided by the filter in response to prevailing lighting conditions and the user's personal taste. Under low glare conditions (e.g., low levels of ambient light), it may be desirable to have a low absorption, high transmission contrast enhancement filter in order to provide truer display color, longer life of the display, and increased brightness. Under high glare conditions, it may be desirable to have a high absorption, low transmission contrast enhancement filter to provide enhanced contrast and reduced glare.
Devices useful as variable transmission filters or attenuators are known in the art. Although the ability to vary the optical density of the device in response to changing ambient lighting conditions would be expected to produce improvement in display readability over that produced by fixed density filters, such variable density devices have a number of shortcomings which lead to disappointing performance. A particularly critical property of displays and display filters is total reflectivity. Although contrast is readily enhanced by attenuators of many types, extremely low reflectivity is required to avoid eye strain and distraction that results from specular and other reflection. User controllable variable transmission devices known in the art result in undesirably high levels of total reflectivity because of the large number of materials, layers, and interfaces present.
For example, a variable density contrast enhancement filter may be provided by an electrochromic device which changes its optical properties upon application of an electric current or potential. U.S. Pat. No. 4,338,000 (Kamimori et al.) reports a variable density electrochromic contrast enhancement filter which can be used as a cover glass panel for a television. PCT published application WO/96/34088 reports an electrochromic device for a cathode ray tube which employs an ambient light sensor to control the attenuation of the device to provide a constant level of contrast. The electrochromic devices typically have a multilayer structure including a layer of an electrically conductive material, an electrode formed from a layer of an electrochromic material, an ion conductive layer, a counterelectrode, and another electrically conductive layer. The large number of interfaces between layers having different refractive indices may lead to a high cumulative level of reflectivity, thereby reducing the amount of light which the device is capable of transmitting and producing less than expected glare reduction.
Other examples of variable transmission devices are known in the art. Devices comprising electroactive chromophores and an electrolyte solution are reported in U.S. Pat. No. 5,801,873 (Byker) and U.S. Pat. No. 5,808,778 (Bauer et al.). Devices comprising suspended particle technology are reported in U.S. Pat. Nos. 1,955,923 (Land) and U.S. Pat. No. 5,130,057 (Saxe). Suspended polyiodide particles are aligned by an electric field in order to increase optical transmission, and allowed to relax in the absence of a field to provide decreased transmission. A monolithic electrochromic device comprising sequentially deposited layers of oxide coatings is disclosed in U.S. Pat. No. 5,404,244 (VanDine et al.). All of these devices produce reflected light from their various components, multiple interfaces, and front and rear surfaces.
Moreover, when variable density filters are used, it is desirable for the variable density portion of the filter not to interfere with other functions typically performed by display filters, such as EM shielding, electrostatic charge dissipation, and front surface antireflection, so that the advantages of these highly useful multifunctional filters can be retained.
In view of the foregoing, a variable transmission contrast enhancement filter having reduced reflectivity is desired.